We've had Tanniks like 3/4 times out of the past 7 maybe, this is getting stupid.
I haven't seen Sepiks for a while, why don't you put that as the NF.
Tanniks is boring now okay
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My first guess is that Taniks and Omnigul are the most annoying Strikes and Bungie wants every Nightfall player to work as hard as possible for their potential rewards. The cynical side of me says it's a dlc thing, they want reward dlc owners and penalise those of you who don't have The Dark Below or House of Wolves.